"""Evaluator for arithmetic AST nodes produced by calc.parser."""

from calc.parser import Num, BinOp, Unary


class EvalError(Exception):
    pass


def evaluate(node) -> int | float:
    """Walk the AST and return the numeric result.

    Whole-valued floats are returned as int (e.g. 4/2 → 2, not 2.0).
    Division by zero raises EvalError.
    """
    if isinstance(node, Num):
        return node.value
    if isinstance(node, Unary):
        operand = evaluate(node.operand)
        if node.op == "-":
            return -operand
        raise EvalError(f"unknown unary operator: {node.op!r}")
    if isinstance(node, BinOp):
        left = evaluate(node.left)
        right = evaluate(node.right)
        if node.op == "+":
            result = left + right
        elif node.op == "-":
            result = left - right
        elif node.op == "*":
            result = left * right
        elif node.op == "/":
            if right == 0:
                raise EvalError("division by zero")
            result = left / right
        else:
            raise EvalError(f"unknown operator: {node.op!r}")
        # Normalize: whole-valued float → int
        if isinstance(result, float) and result == int(result):
            return int(result)
        return result
    raise EvalError(f"unknown node type: {type(node).__name__}")